Centrifugal Pump: Inadequate Performance. What happened?

[RefineryRR]

I have a vertically-submerged 5-stage centrifugal pump that suddenly yielded reduced head (and therefore reduced capacity). The pump had a normal operating differential head of 22.5 bar at a flow of 80 m3/h, but the pump was delivering only around 20 bar diff head and a flow of around 60m3/h.
We confirmed that the operating temperature was within spec. Speed was confirmed (despite being motor-driven). The recorded suction pressure was 0.7 bar (minimum for the pump according to the data sheet is 0.4 bar). We went through the entire system and matched everything (levels, tower operating pressures, etc.) to the run-period when the pump was delivering as per design - but the pump continued to not deliver.

Eventually, we discovered that there was an obstruction in the suction line of the pump. Once that was removed, the suction pressure rose to 0.75 - 0.8 bar, and the pump was delivering as per design.

What, exactly, happened in this system? What were the hydraulic effects at play to cause this phenomenon? No-one in my team can reconcile the findings with the pump's delivery issues.

 

[LittleInch]

As i noted above, the min inlet for the fluid is measured in m head. The conversion to predate only works for a particular fluid at a particular temperature with a set vapour pressure.

Change any of those and your min pressure goes out of the window.

Cavitation often occurs at heads higher than the NPSHR. An increase of a few metres will make all the difference.

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